The combustion of fossil fuels in nearly pure oxygen, rather than air, presents an opportunity to simplify carbon dioxide (CO2) capture in power plant applications. Oxy-combustion power production provides oxygen to the combustion process by separating oxygen from air. However, the capital cost, energy consumption, and operational challenges of oxygen separation are a primary challenge of cost-competitive oxy-combustion systems. Oxy-combustion system performance can be improved by two means: (1) by lowering the cost of oxygen supplied to the system and (2) by increasing the overall system efficiency. The R&D within the Advanced Combustion Systems Program is aimed at strategies to improve oxy-combustion system efficiency and reduce capital cost, offsetting the challenges of oxygen production.

In an oxy-combustion process, a pure or enriched oxygen (O2) stream is used instead of air for combustion. In this process, almost all of the nitrogen (N2) is removed from the air, yielding a stream that is approximately 95 percent O2. Hence, the volume of flue gas, which is approximately 70 percent carbon dioxide (CO2) by volume, from oxy-combustion is approximately 75 percent less than from air-fired combustion. The lower gas volume also allows easier removal of the pollutants (SOx, NOx, mercury, particulates) from the flue gas. Another benefit is that because N2 is removed from the air, NOx production is greatly reduced.

Alstom's 3-MWth Boiler Simulation Facility

Oxy-combustion power production involves three major components: oxygen production (air separation unit [ASU]), the oxy-combustion boiler (fuel conversion [combustion] unit), and CO2 purification and compression. These components, along with different design options, are shown below. Oxy-combustion systems can be configured differently with these components, resulting in different energetic and economic performances.

Oxy-combustion systems can be configured in either low- or high-temperature boiler designs. In low-temperature designs, flame temperatures are similar to that of air-fired combustion (~3,000 °F), while flame temperatures exceed 4,500 °F in the advanced high-temperature design. Low-temperature designs for new or retrofit applications recycle combustion products to lower the flame temperature to approximate the heat transfer characteristics of air-fired boilers. High-temperature designs use increased radiant heat transfer in new construction applications to reduce the size and capital cost of the boiler.

Today’s state-of-the-art oxy-combustion systems would use a cryogenic process to supply O2, atmospheric-pressure combustion for fuel conversion in a conventional supercritical pulverized-coal boiler; substantial flue gas recycle; conventional pollution control technologies for SOx, NOx, mercury, and particulates; and mechanical CO2 compression. However, the costs of today’s oxy-combustion technologies are too high. The Advanced Combustion Systems R&D Program is developing advanced technologies to reduce the costs associated with current systems. R&D efforts are focused on developing pressurized oxy-combustion power generation systems, as well as membrane-based oxygen separation technologies. Currently, NETL supports three oxy-combustion projects in collaboration with industry and academia, including projects funded by American Recovery and Reinvestment Act of 2009 (ARRA), two at pilot scale, and one at bench scale. These projects are focused on verifying system design and operation concepts.